Such measurement sites are, for example, pH measurement sites, or other measurement sites for analytical measurement. This can include, in particular, other potentiometric, amperometric, coulometric, colorimetric, photometric, turbidimetric, and spectrometric sensor units.
A sensor unit comprises at least one transducer that emits an electrical signal dependent upon the value of the measurand. Frequently, the transmission function of the transducer is variable. This holds true, especially, for pH sensors. Consequently, pH sensors must be serviced at appropriate time points—especially, recalibrated. The length of the time intervals between the maintenance measures or the calibration intervals strongly depends upon environmental influences to which the sensor is exposed over its lifespan. The overall life expectancy is also strongly influenced by these environmental conditions (hereinafter also termed the load or sensor load).
In pH measurement and also in analytical measurement, the term, “calibration,” is frequently used differently than is customary. Generally, a calibration is understood to be the check of the display of a measuring device against a standard, and the deviation between the true value and displayed value is identified. Matching the displayed value with the true value is termed adjustment. The measure that is normally termed calibration in pH measurement and analytical measurement is, strictly speaking, an adjustment. Since this language is, however, routine in these fields, it will also be used here and in the following.
In the prior art, a plurality of measures is known for equipping sensors with intelligent self-diagnosis. There is a series of approaches for predicting a time point at which a maintenance measure of a sensor unit should be performed. For example, an adaptive calibration timer is known from DE 101 41 408 A1 that iteratively adapts a specified basic calibration interval for a pH sensor as a function of the temperature and pH load to which the sensor unit is exposed. A method is known from WO 2004/025223 A2 by means of which a residual life of the sensor unit is forecast from an extrapolation of the calibration parameters of the pH sensor unit determined repeatedly in the past, i.e., the zero point and slope of the sensor characteristic. A method is known from DE 10 2004 012420 A1 in which a history of the pH and temperature load that a sensor unit experiences over its operating life is saved in the form of a load matrix, and displayed. Operating hours meters are also known that record and add up the operating hours during which a sensor unit has been exposed to a temperature above a specific threshold, or to a pH below or above a specific threshold.
Today, operating units such as measuring transducers or portable operating devices that can be connected by wire or wirelessly to a sensor unit for communication and can be used at measurement sites in analytical engineering—especially in the processing industry—generally offer several such diagnostic functions. They are to help a user schedule the maintenance of the individual sensor units, so that maintenance measures can be performed in a timely manner, before the respective sensor units no longer provide reliable measured values, and so that set maintenance intervals are not exceeded. However, a user requires a lot of experience to interpret the various diagnostic results, which are generally presented on several displays which are separate from each other, such as in the form of several bar charts and possibly additional displayed numeric values, and to relate them to each other to achieve this goal.